The "count" object can be used only in "resource" and "data" blocks, and only when the "count" argument is set

Terraform
1

I am building ec2 instances using passing count variable to pass number ec2 instances. Part of the code when i am trying add storage referencing the instance i am getting following error. How should we refer instance_id in volume attachment resource ?

resource "aws_ebs_volume" "Vol1" {
  availability_zone = "us-east1a"
  size              = 10
  encrypted         = "true"
}
resource "aws_volume_attachment" "install" {
  device_name ="/dev/sdf"
  volume_id= aws_ebs_volume.Vol1.id
  instance_id = "${aws_instance.tf_instance.*.id[count.index]}"
}

The “count” object can be used only in “resource” and “data” blocks, and only
when the “count” argument is set.

Can some one please help.